Nanostructurated membranes for the microbiological purification of drinking water

introduction
The purification of water intended for human consumption consists in the elimination of various pollutants such as chemical substances (i.e. pollutants, toxic metals), biological pollutants (algae, bacteria, fungi, parasites, viruses), suspended solids and gases.

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There are several methods used in the water purification process, including: (1) physical processes, such as filtration, sedimentation or distillation; (2) biological processes, such as sand filters, activated carbon; (3) chemical processes, such as flocculation, chlorination, use of ultraviolet light.

As for the physical approaches used in water purification, they differ in the phenomenon on which the processes are based. For example, filtration is based on the separation of solids from fluids, interposing a porous medium (filter) which retains the solid particles and allows the fluid to pass to the other side. On the other hand, the sedimentation approach uses the gravitational force that causes the solids to form a deposit at the bottom of the tube that contains contaminated water, while distillation involves the transformation of the liquid (water) into a vapor phase, a process that it is based on the difference in volatility of the compounds.

Slow sand filters are an example of biological approaches used in water purification, which involves the use of 1 to 2 m deep pipes filled with sand, which retain the impurities present in the filtered water. Activated carbon (charcoal), another method used, is a microporous carbon with a large surface area and better adsorption properties.

In scientific terms, flocculation is a process in which suspended colloids are destabilized after the addition of a clarifying agent; As for the water purification process, the phenomenon of flocculation can refer to the destabilization and coagulation of the contaminants present in the water. Water chlorination is commonly found in water supply treatment and involves the addition of chlorine or hypochlorite to kill microbes and prevent the spread of waterborne diseases.

The use of electromagnetic light, especially with a short wavelength (in the ultraviolet range), is commonly used in disinfection, because it produces gaps in the structure of the nucleic acids of microorganisms, disabling cell functions. This method is often used in water decontamination.

This chapter mainly focuses on the most researched physical methods involving the filtration process. Compared to other approaches used in water treatment, membrane water filtration has some advantages, such as: (1) continuity of operation; (2) does not require the use of chemicals; (3) does not involve high energy consumption; (4) the possibility of scaling, integrating / integrating other processes and the possibility of automation (Street et al., 2014).

There are several parameters that influence the properties and efficiency of membranes used in water filtration: (1) the size of the pores in the membrane / the size of the polluting molecules / particles; (2) the positive / negative charge of the membrane surface, respectively the polarity of the polluting molecules; and (3) the adsorption capacity of the membrane surface (Street et al., 2014).

Depending on the pore size of the filters (membranes) used, it is possible to remove contaminants of different sizes and achieve better purification by reducing the size of the pores; therefore, the methods available are: (1) microfiltration (pore size of about 0.1 µm), which removes bacteria and solids suspended in the water; (2) ultrafiltration (pore size of about 0.01 µm), which in addition to microfiltration removes viruses; (3) nanofiltration (pore size of about 0.001 µm), which removes most of the organic molecules and some multivalent ions (divalent ions from hard water); and (4) reverse osmosis (approximately 0.0001 μm pore size), which removes all organic and mineral molecules in the water, producing ultrapure water

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